Kinetic Oxygen Isotope Fractionation between Water and Aqueous OH- during Hydroxylation of CO2

The fractionation of stable oxygen isotopes between water and aqueous hydroxide ions (epsilon(18)(H2O/OH-)) is an integral parameter in chemical modeling. Quantum chemical calculations predict thermodynamic isotope equilibrium epsilon(18)(H2O/OH-) values that are ca. 24 parts per thousand lower than what laboratory experiments suggest. Here, we performed quantitative BaCO3 precipitation experiments across a wide range of temperatures (1-80 degrees C) to complement the limited set of existing experimental data. The known isotope compositions of the tank CO2 gas and the hyperalkaline Ba(OH)(2) solutions allowed us to calculate apparent isotope fractionations. Our data broadly agree with previous experimental epsilon(18)(H2O/OH-) estimates and show a temperature dependence similar to what theoretical models predict (N = 20; T is in degrees C): epsilon(18)(H2O/OH-) = -0.035(+/- 0.005) x T + 43.4(+/- 0.2). The small difference (-0.6 to 0 parts per thousand) between the delta C-13 values of the precipitates and the tank CO2 suggests that kinetic isotope effects (KIEs) related to the preferential adsorption of isotopically light CO2 are insignificant. Instead, we argue that the observed 24 parts per thousand offset between the experimental and theoretical values derives from KIEs related to the preferential reaction of isotopically light OH- during CO2(aq) hydroxylation (KIEOH-). In our experimental setup, dehydroxylation is posited to be slower than carbonate precipitation; therefore, we conclude that our KIEOH- estimate is close to its maximum value expected for a purely unidirectional reaction path. In natural systems, however, isotope equilibration between water and dissolved inorganic carbon species generally reduces the extent of KIEOH- inherited by the solid carbonate